1. Field of the Invention
The invention relates generally to techniques for generating polarized light and, more particularly, to techniques for generating vector beams.
2. Background of Related Art
As known from theory and experiment, focusing systems are in general limited in their ability to focus light. This is commonly referred to as “diffraction limited focusing” and is described by conventional Abbe Diffraction theory. This conventional theory does not consider the polarization properties of the light field and their consequence on focusing. Focusing systems (e.g. microscope objectives) may be able to tightly confine focused light in a transverse focal (i.e. XY-) plane, but are less able to achieve the same spatial resolution in the axial (or Z-) direction. For many applications, this elongated focus is of little import. For other applications, however, in particular applications with features on the sub-micron and nanometer scale, this effect is quite problematic.
Recent advances in interferometric fluorescence microscopy, such as “4-Pi” confocal microscopy, have enabled overall resolution enhancements beyond the diffraction limitation mentioned above. These techniques achieve focusing resolutions known as “superresolution.” Most of these techniques rely exclusively on the use of scalar polarized beams, i.e., beams that have polarizations that are uniform over their cross section. Even these techniques, however, are limited in the tightness of the focus they can achieve. As such, some have proposed using vector beams, i.e., beams that exhibit spatially nonuniform polarization.
Two types of vector beams, radially- and azimuthally-polarized light, have been proposed in theory. Such beams preserve the axial symmetry of an optical system, and are sometimes referred to as “doughnut” or “cylindrical beams” because of the phase singularity resulting in an on-axis null intensity. When focused with a high numerical-aperture (NA) lens, the radially-polarized beam may produce a predominantly longitudinally polarized (i.e., on-axis) electric field component in the focal region. An azimuthally polarized beam focused by a high-NA lens may produce a strong on-axis magnetic field component and a purely transverse electric field.
Heretofore, the techniques for forming vector beam polarized light have been limited in design and in effectiveness. One challenge to the utilization of vector beams has been the robustness of the method of generation. Separate approaches based on Mach-Zehnder interferometry or a modified laser resonator have been demonstrated, but these require costly, active phase stabilization techniques. Alternatively, a spatial light modulator (SLM) has been proposed, but that too is costly and requires the computation of a complex algorithm for the desired phase function. Also SLM's can result in undesired diffraction effects due to pixelation. Another method uses circularly polarized light and space-variant subwavelength gratings to create radially and azimuthally polarized light. This approach, however, has only demonstrated vector beams at a wavelength of 10.6 μm and has not been used to form frequencies in the visible region. Yet, another method uses specially-designed twisted nematic liquid crystal polarizers to generate radially and azimuthally polarized light. This technique has been used at visible frequencies, but is quite costly and cumbersome to fabricate.